This invention relates to a method of fabricating a semiconductor device having an aluminum isolation region and more particularly to a method of selective formation of an aluminum-diffused region within a silicon semiconductor substrate.
Aluminium is a P-type dopant having a large diffusion coefficient in silicon and is therefore highly effective for forming a deeply-diffused region such as for example a through-diffused region for isolation purposes. A method has hitherto been known in which aluminum diffusion source layers in the form of a predetermined pattern are formed on the major surface of a silicon semiconductor substrate by means of deposition or the like, and the silicon semiconductor substrate is then heated in an atmosphere including an oxygen gas to form silicon-aluminum alloy layers from which aluminum diffuses into the silicon semiconductor substrate to deeply and selectively form aluminum diffused layers.
The method as mentioned above is described in, for example, U.S. Pat. No. 4,040,878 issued to Rowe on Aug. 9, 1977 and U.S. Pat. No. 4,066,485 issued to Rosnowski et al. on Jan. 3, 1978.
In the conventional method, the oxygen gas contained in the diffusing atmosphere reacts with exposed portions of a silicon substrate to form an SiO.sub.2 film and with aluminum deposited on the substrate to form Al.sub.2 O.sub.3 films. If the thus formed SiO.sub.2 film has a sufficient thickness, it may prevent out-diffused impurity atoms from autodoping, in other words, backdoping into the substrate through the SiO.sub.2 film. The Al.sub.2 O.sub.3 films on the aluminum prevent aluminum from evaporation.
In carrying out the process of the prior art, the accurate controlling of oxidizing conditions including a flow rate of the oxygen gas is important to obtain an accurate diffusion front and to obtain high reproducibility of semiconductor devices. Further, it is difficult in the prior art to obtain such a thick SiO.sub.2 film that completely prevents the autodoping avoiding introduction of cracks in the SiO.sub.2 film, because a thick SiO.sub.2 film, which is formed during the aluminum diffusion in an oxygen containing atmosphere is easily crystallized. The crystallized SiO.sub.2 is never acceptable to the semiconductor devices.